The objective is to advance our understanding of the neuroanatomical organization of descending auditory pathways acting at the level of the cochlea and cochlear nucleus in animals, including humans. Relying on light and electron microscopic techniques for tracing neuronal connections and on the selective marking of neurons and fibers by immunocytochemical means, the specific aims of the project are: 1) to determine if differences exist in the peripheral projections of lateral vs. medial olivocochlear (OC) neurons in the rat and guinea pig, both of which species offer certain practical anatomical advantages for OC investigation are widely used in studies of hearing; 2) to map and count OC neurons in the human brainstem using acetylcholinesterase histochemistry and antisera against certain neurotransmitter-related compounds associated with one or both systems in mammals, and by these means determine if two efferent cell populations are present; 3) to map in detail the numerically important descending projections from lateral vs. medial regions of the superior olivary complex back to the cochlear nucleus; and 4) to determine the identify and distribution of ascending neurons in the cochlear nucleus which innervate neurons projecting back to the cochlear nucleus or cochlea. Since the morphology of the cells, tracts, and connections of a neuronal system constitute fundamental constraints on its functional capabilities, answers to the questions posed in these studies will have both an immediate and long-term impact on the way in which basic and clinical scientists conceptualize, investigate, and model the functions of the ascending and descending pathways in both normal and impaired listeners.